The present invention relates to systems for transmitting data via telephone lines, particularly automated systems which share a telephone line with other users.
Systems using consumer telephone lines for exchanging data with remote locations have recently been proposed. One specific application of such systems is the transmission of utility meter readings and thermostat settings from homes to a central data collection station, thereby eliminating the need for personal visits by meter readers. In such systems, it is also possible to transmit from the central station to the homes information about time-related energy costs, thereby enabling the energy usage in the home to be adjusted, either manually or automatically, to minimize energy costs. Such information would be particularly useful if electrical utilities should adopt the practice of varying their kilowatt-hour charge as a function of total demand, maximum rates being charged during periods of peak demand and lower rates being charged during periods of reduced demand.
Particularly in the case of home installations, such systems would require access to a telephone line for only brief, widely spaced periods of time. For example, if the system is arranged only to perform meter readings, data transfer might occur only once or twice each month, with each transfer operation requiring access to the telephone line for a period of several minutes. Under these circumstances, it would not be economically feasible for the electrical power utility to have a separate telephone line connected to each home.
However, if data transfer is effected via a telephone line already connected to the home, it would be highly desirable, if not indispensable, that the data transfer system not interfere with normal use of the telephone line by individuals in the home.